Owned the 300 f4 IS plus a 1.4 extender. It was a nice lens without using the extender. Never as long as I liked. I picked up the 400 5.6 a year later. Really good IQ if enough light was available. I traded the 300 as it just sat idle for several years.

400 5.6 for sure. 1/800 and up (or is it down - back to FM summer school for me) shutter speed. I bought a small canvas bag and plastic pellets at Hobby Lobby that works great as a bean bag on most any available object, with this, tripod is optional.

Let me be in the minority, I like the 300s with TCs for the flexablity on budget. Unless you have to optimize on AF and I hear that 400 f5.6 is better than the 420 f5.6 IS, I would suggest getting 300 F4 IS. In super tele range, IS comes in handy and if you are on a trail, you are not always taking your 70-200, and 300 F4 IS may come in handy both for focal length and additional stop. I was planning on selling the 300 when I got its bigger f2.8 bro and potentially trading in a 400, but its a keeper for the flexiblity. Couple of shots:

Neither. The 100-400 is far more flexible and just as good. Good mfd, image stabilisation, zoom. If you typically shoot in less than ideal light the image stabilisation is going to result in far more keepers.

400 5.6 but the 300mm has its place for backyard birding or Hummingbirds. I have a freidn that shoots hummers only with the 300 F4 IS specifically because he lives in crap light most of the time and the IS comes in handy and is a great focal length for Hummers within 10ft or so sitting in a blind. I have the 400 but I live in an area that has great light most of the time alloing me to have high shutter speeds even at F8-F11 with a TC. OTherwise I would of bought the 300. Now the 300 2.8 mkII, that is a lens I do wants to haz. Too rich for my blod though.

I've owned both, but sold the 300 to get the 400 for my 7D.
Most of my BIF shots are handheld. I don't have a gimbal setup.
With decent shuttle speeds 1/1000 or faster if you can, you can get solid results.

I set custom 1 position to 400iso/1/1000th/5.6 with 8fps continuous frames, and expanded focus point, so I just twist that dial and I'm in the ballpark.

No ANgle wrote:
400 5.6 but the 300mm has its place for backyard birding or Hummingbirds. I have a freidn that shoots hummers only with the 300 F4 IS specifically because he lives in crap light most of the time and the IS comes in handy and is a great focal length for Hummers within 10ft or so sitting in a blind. I have the 400 but I live in an area that has great light most of the time alloing me to have high shutter speeds even at F8-F11 with a TC. OTherwise I would of bought the 300. Now the 300 2.8 mkII, that is a lens I do wants to haz. Too rich for my blod though....Show more →

This is a good point. In low light the 300mm is an excellent choice. Several years ago I took both the 300mm and 400mm f/5.6 into the jungle in Eastern Ecuador. The 400mm was completely unused because I could not achieve shutter speeds high enough to prevent motion blur. Moreover, the 300mm does have great macro ability and that should not be overlooked because it offers tremendous utility for closeup shots of insects etc...

dmcharg wrote:
Neither. The 100-400 is far more flexible and just as good. Good mfd, image stabilisation, zoom. If you typically shoot in less than ideal light the image stabilisation is going to result in far more keepers.

I'd vote for 400/5.6, you can always use your 70-200 or 70-200 + TC for shorter fl.

For support and trekking I'd recommend a carbon tripod with a monopod head (not a ball head) I find it easier to use with long lenses. But you might want to first try your lens and then decide if you want to invest in a monopod or tripod